This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Graft-versus-host disease (GVHD) represents a major cause of morbidity and mortality following allogeneic hematopoietic stem cell transplantation (HSCT). In an effort to permit the safe engraftment without GVHD, much effort has focused on testing the GVHD inducing capability in different T cell subsets. Among various hypothesis and methods examined, separation of GVHD inducing T cells based on their differentation status and/or tissue-specific homing receptors is an attractive strategy. In this study, we plan to enumerate naive-memory T cells and gut-specific integrin a4B7+ T cells in allogeneic peripheral stem cell grafts, and correlate with the incidence, severity and site of GVHD post-transplant. We also plan to assess T cell recovery post-transplant with respect to their differentiation status and tissue specific integrin. We plan to examine patients undergoing HSCT from a sibling donor.